According to the couple's official spokesman, the balance of their feelings about what has happened to the four-year-old has tipped towards accepting the worst possible outcome, nearly six months after she vanished.

"Kate and Gerry are realistic enough to know that there is a probability she is dead," Clarence Mitchell said.

"They have not given up hope that she could still be alive and is being looked after somewhere.

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"And they cling to that and have not given up, but human nature is that you always fear the worst and they need to know what has happened.

"This open-uncertainty cannot hang over them for the rest of their lives."

The sad milestone came as Kate McCann's mother admitted her daughter and son-in-law made a "terrible mistake" to leave Madeleine and her twin siblings alone, and said the repercussions had ripped their "perfect" family apart.

But Mrs Healy told the Liverpool Echo that news about renewed police searches in Portugal, and the appearance of police officers at a reservoir to the north of the Algarve resort town of Praia da Luz, left her "scared" that Madeleine's body would be found.

Mrs Healy said her daughter was tormented by the "scurrilous rubbish" about her and Gerry's culpability in Madeleine's disappearance that is reported daily in the Portuguese press and repeated in the UK.

She said the couple continue to have counselling and have relaunched a publicity campaign in the UK and Portugal in a bid to find their daughter.

But she Mrs Healy said the strain of the case was taking its toll.

"They have to keep trying to get their daughter back - that overrides everything else they are feeling," she said.

Madeleine disappeared from the McCann's rented apartment in Praia da Luz on May 3.